What does the letter "R" mean in trench sizes?

I was looking coats in www.burberry.com, but the model I am interested in has sizes like UK 4R, 6R, 8R etc. What does that R mean? I do not see it anywhere in the webpage..

here is an example with the letter:
http://www.burberry.eu/fcp/product/...TTON-TRENCH-COAT/10000008526?colour=jet_black

and by the way, would that be a good choice for black summer trench?

if it's anything like the guy's section...R means regular (i.e., sleeves fit the arm lengths of an average sized person). S is for short and L is for Long. R is what most things are made in.